AI becoming sentient plausibility?
There are already programs that have broken free of our control (Morris worm) so that in itself doesn't imply any great computational demands.

Sentience is ill-defined but is certainly not a pre-requisite for a program to do mischief beyond what its creators intend.

It's difficulty to estimate what sort of processing power is required to support human-like intelligence, since we don't know what the most efficient way to achieve that would be.  If the most processing efficient would be to implement a neural network approaching the number of neurons and interconnects of the human brain processing signals at the same rate, the fastest artificial neural network implementations extant are at least 4-5 orders of magnitude short, is thousands of times less power efficient, and doesn't seem to have a realistic way to scale to the number of interconnects required (see this question)